Transmitters and receivers in many communication systems employ gain control techniques to adjust the signal levels of various amplifier stages. Analog and digital automatic gain control (AGC) circuits are well-known in the art. For example, AGC circuits are frequently used in transmitters or receivers in the field of cellular, wireless, and spread-spectrum wireless communication systems.
The AGC function may be employed to bring a newly acquired signal at the input of the receiver into the dynamic range of the receiver when a communication link is first established. Thereafter, the AGC function typically continually adjusts the overall receiver gain to compensate for fluctuations in received signal strength associated with fading, interference, periods of “silence” between data frames, or similar interruptions of the signal level.
Such gain control circuitry is often employed to maintain a constant level in the front end of a receiver, including the amplification and RF/IF demodulation stages. The constant level is maintained such that the output (demodulated) baseband signal level of the receiver remains within well-defined limits, even though many factors may vary gain levels within each component of the front end.
Typically, gain control problems were solved using analog receive signal strength detectors, rectification and averaging techniques, or other control loops based on signal amplitude or energy averaging. Receive signal strength detectors, however, require expensive circuits and control loops that are prone to steady state oscillation. A need therefore exists for improved methods and apparatus for automatic gain control. A further need exists for techniques for adjusting the gain of a received signal based on received signal envelope detection.